Inhibition of cyclooxygenase (COX) enzymes is believed to be at least the primary mechanism by which nonsteroidal anti-inflammatory drugs (NSAIDs) exert their characteristic anti-inflammatory, antipyretic and analgesic effects, through inhibition of prostaglandin synthesis. Conventional NSAIDs such as ketorolac, diclofenac, naproxen and salts thereof inhibit both the constitutively expressed COX-1 and the inflammation-associated or inducible COX-2 isoforms of cyclooxygenase at therapeutic doses. Inhibition of COX-1, which produces prostaglandins that are necessary for normal cell function, appears to account for certain adverse side effects that have been associated with use of conventional NSAIDs. By contrast, selective inhibition of COX-2 without substantial inhibition of COX-1 leads to anti-inflammatory, antipyretic, analgesic and other useful therapeutic effects while minimizing or eliminating such adverse side effects. Selective COX-2 inhibitory drugs such as celecoxib and rofecoxib, first commercially available in 1999, have therefore represented a major advance in the art. These drugs are formulated in a variety of orally deliverable dosage forms.
Parenteral routes of administration, including subcutaneous, intramuscular and intravenous injection, offer numerous benefits over oral delivery in particular situations, for a wide variety of drugs. For example, parenteral administration of a drug typically results in attainment of a therapeutically effective blood serum concentration of the drug in a shorter time than is achievable by oral administration. This is especially true of intravenous injection, whereby the drug is placed directly in the bloodstream. Parenteral administration also results in more predictable blood serum concentrations of the drug, because losses in the gastrointestinal tract due to metabolism, binding to food and other causes are eliminated. For similar reasons, parenteral administration often permits dose reduction. Parenteral administration is generally the preferred method of drug delivery in emergency situations, and is also useful in treating subjects who are uncooperative, unconscious, or otherwise unable or unwilling to accept oral medication.
Relatively few NSAIDs are commercially available in injectable form. Non-selective NSAIDs such as ketorolac tromethamine salt that are available for parenteral use are effective analgesics but have been associated with side effects typical of such non-selective NSAIDs. These side effects have included upper gastrointestinal tract ulceration and bleeding, particularly in elderly subjects; reduced renal function, potentially leading to fluid retention and exacerbation of hypertension; and inhibition of platelet function, potentially predisposing the subject to increased bleeding, for example during surgery. Such side effects have seriously limited the use of parenteral formulations of non-selective NSAIDs.
It would therefore represent a further significant advance in the art if a parenterally deliverable formulation of a selective COX-2 inhibitory drug could be provided.
It is known to prepare parenteral formulations by a process of lyophilization (freeze-drying) of an aqueous solution of the therapeutic agent. See for example Remington: The Science and Practice of Pharmacy, 19th edition (1995), Mack Publishing, pp. 1544-1546. According to Remington, excipients often are added to the therapeutic agent to increase the amount of solids, so that the resulting powder is more readily visible when the amount of the therapeutic agent is very small. “Some consider it ideal for the dried-product plug to occupy essentially the same volume as that of the original solution. To achieve this, the solids content of the original product must be between approximately 5 and 25%. Among the substances found most useful for this purpose, usually as a combination, are sodium or potassium phosphates, citric acid, tartaric acid, gelatin and carbohydrates such as dextrose, mannitol and dextran.” Remington, loc. cit.
Parecoxib, disclosed in U.S. Pat. No. 5,932,598 to Talley et al., is one of a class of water-soluble prodrugs of selective COX-2 inhibitory drugs. Parecoxib rapidly converts to the substantially water-insoluble selective COX-2 inhibitory drug valdecoxib following administration to a subject. Parecoxib also converts to valdecoxib upon exposure to water, for example upon dissolution in water. The high water solubility of parecoxib, particularly of salts of parecoxib such as the sodium salt, by comparison with most selective COX-2 inhibitory drugs such as celecoxib and valdecoxib, has led to interest in developing parecoxib for parenteral use. Parecoxib, having the structural formula (I) below, itself shows weak in vitro inhibitory activity against both COX-1 and COX-2, while valdecoxib (II) has strong inhibitory activity against COX-2 but is a weak inhibitor of COX-1.

Other water-soluble selective COX-2 inhibitory drugs and prodrugs are known. For example, U.S. Pat. No. 6,034,256 to Carter et al. discloses a series of water-soluble benzopyrans said to be useful as selective COX-2 inhibitory drugs, including the compound (S)-6,8-dichloro-2-(trifluoromethyl)-2H-1-benzopyran-3-carboxylic acid (III) and salts thereof.

While these and other selective COX-2 inhibitory drugs and prodrugs have been proposed in general terms for parenteral administration, no pharmaceutically acceptable injectable formulation of such drugs or prodrugs has hitherto been described. As will be clear from the disclosure that follows, numerous problems have beset the formulator attempting to prepare such a formulation, illustratively of parecoxib. The present invention provides a solution to these problems.